A photocatalyst is a substance that makes a chemical reaction happen more quickly under light irradiation without being changed itself. Under light irradiation, the photocatalyst can activate adsorbed oxygen and moisture on the surface of a material, thereby producing free hydroxy groups and active oxygen that have a very strong oxidizing ability and setting off an oxidation reaction, so as to completely decompose organic matter, bacteria, viruses, etc. into carbon dioxide and water.
The photocatalytic air purification technology is currently an ideal technology internationally acknowledged for treatment of environmental pollutants. The key to a desirable photocatalytic purification effect is to further enlarge a contact area between polluted air and a photocatalyst and further ensure that the surface of the photocatalyst receives sufficient light irradiation. Moreover, in addition to a photocatalyst that is activated by irradiation of ultraviolet light, the photocatalyst may alternatively be a photocatalyst that is activated by irradiation of other light (for example, visible light).
In addition, existing photocatalytic purification apparatuses are mainly classified according to structures into the following types of a single-channel type, a single-channel surface-extended type, a honeycomb-channel stacked type, and a light-source-extended type. Each type of apparatus is described below in brief.
(1) Single-channel Type
A single-channel structure is a typical structure of a photocatalyst apparatus. A single-channel photocatalyst apparatus comprises only one channel. A photocatalyst coating is applied to a wall surface of the channel. An ultraviolet lamp is disposed at the centre of the channel. A photocatalyst is activated to set off a photocatalytic oxidation reaction.
JIN Ning et al. from University of Shanghai for Science and Technology researched an apparatus that integrates pre-filtration, high-efficiency filtration, active carbon filtration and photocatalytic filtration. The structure of such an integrated apparatus 1A is shown in FIG. 1. Air enters the apparatus through a sidewall 10A1 of a cylindrical barrel 10A. Under the action of a fan 20A disposed inside, the air sequentially passes through multiple filter meshes (a pre-filter mesh 30A, a high-efficiency particulate air (HEPA) filter mesh 40A, and an active carbon filter mesh 50A) to reach an internal filter mesh 60A with TiO2 (a photocatalyst) attached thereto. A double-tube ultraviolet lamp 70A is disposed at the centre of the cylindrical barrel 10A of the integrated apparatus 1A. Therefore, when the photocatalyst is in an activated state and polluted air passes through the photocatalyst, a redox reaction occurs. In this way, an air purification process is implemented as a whole by multi-filtration.
However, as shown in FIG. 1, the single-channel type has a disadvantage of a small contact area between polluted air and the photocatalyst (TiO2). Therefore, a reaction area where the reaction takes place is also relatively small.
(2) Single-channel Surface-extended Type
As discussed above, because a single-channel photocatalyst apparatus has a relatively small reaction area, to increase a reaction area between polluted air and a photocatalyst, a single-channel surface-extended structure is proposed. The single-channel surface-extended structure is based on the single-channel structure and replaces the simple wall surface of a channel with a carrier in other forms as an attachment surface for a photocatalyst. In such a manner, a surface area of the photocatalyst is increased, and thus the reaction efficiency of the apparatus can be improved.
YAN Qinian et al. researched a photocatalyst apparatus in a guided convoluted air-channel form. The structure of such a photocatalyst apparatus 1B in a guided convoluted air-channel form is shown in FIG. 2. Compared with the single-channel photocatalyst apparatus, such a photocatalyst apparatus 1B in the form of a guided convoluted air-channel has a separator plate (not shown) to form a spiral channel 20B. A TiO2 coating (a photocatalyst coating) is sprayed onto a pipe wall 11B and is also sprayed and attached onto the separator plate, thereby increasing a reaction area. Four (only two are shown here) ultraviolet lamp tubes 31A and 32A having different radiation wavelengths are disposed in a central-axis direction of the apparatus. In this way, a resonant light source is formed, to enable a photocatalyst to be in a high-intensity activated state. In the photocatalyst apparatus 1B in a guided convoluted air-channel form shown in FIG. 2, since both the length of the channel and a coating area are considerably extended, the two factors also become the key to the improvement of reaction efficiency.
In addition, ZHAO Gang designed a mesh-type photocatalyst apparatus for passenger trains, and the structure thereof is shown in FIG. 3. Such a mesh-type photocatalyst apparatus 1C is installed on an air return pipe (not shown) and is of a flat drawer-form structure. A distance between an outlet and an inlet of a channel is relatively small, and two end surfaces are each provided with a metal mesh structure. A TiO2 coating (a photocatalyst coating) is attached to the mesh, and is brought into an activated state after being irradiated by two ultraviolet lamp tubes 10C disposed in the middle of the channel. Because of a special shape requirement of this apparatus, the coating of the apparatus is disposed in a direction perpendicular to the channel. Further, to prevent the channel from being blocked, a mesh structure is used. However, the mesh structure does not extend an area as much as expected, which is not an ideal carrier form. As a result, the efficiency of a single-pass reaction of the apparatus is relatively low. Therefore, the structure of the mesh photocatalyst apparatus 1C shown in FIG. 3 is applicable only to a case in which air can repeatedly return and the concentration of polluted air is relatively low.
In addition, SHAN Xinggang et al. conducted experimental research on photocatalysis. An experimental apparatus used in the research is a structural form of a glass sleeve. An ultraviolet lamp tube is installed in an inner quartz tube, and small glass beads are filled in the outer glass sleeve. TiO2 (a photocatalyst) is sprayed and attached onto the small glass beads in a coating form, so as to form a packed bed. In this way, a reaction area can be greatly extended.
However, the single-channel surface-extended photocatalyst apparatuses in the foregoing three cases all have the following disadvantage: air flowing in the channel is subject to relatively large flow resistance, which probably cannot satisfy flow resistance requirements in the field of ventilation systems that are relatively sensitive to flow resistance, for example, a ventilation system for a vehicle.
(3) Honeycomb-channel Stacked Type
Similar to a surface-extended structure, the honeycomb channel structure is proposed to increase a reaction surface. However, because honeycomb channels are distributed intensively and an ultraviolet source cannot be disposed inside each channel, an ultraviolet lamp tube can only be placed at inlets and outlets of the channels. In addition, in consideration of a limited irradiation range of light rays, the channels cannot be configured to be excessively long. Therefore, a honeycomb-channel stacked structure emerges, in which relatively short honeycomb channel structures and ultraviolet lamps are stacked.
A photocatalyst apparatus 1D designed for a passenger cabin by WANG Jun's research group from Beihang University is exactly of such a honeycomb-channel stacked structure. The structure thereof is shown in FIG. 4. The reason for using a honeycomb channel is that, as compared with structures such as in the form of a packed bed and a metal mesh shown in FIG. 3, a honeycomb channel structure has much lower flow resistance. In addition, a TiO2 coating (a photocatalyst coating) 30D is sprayed and attached onto a wall surface of each honeycomb channel 10D. A stacked structure of multiple groups of ultraviolet lamps 20D and honeycomb channels 10D is used, such that most of the honeycomb channels 10D are in desirable conditions of ultraviolet light irradiation. In this way, considerably high reaction efficiency can be achieved.
In addition, LU Yuanwei et al. also researched such a honeycomb-channel stacked structure. A honeycomb channel board used has a size of 300 mm×300 mm, a thickness of 6 mm, and a honeycomb cell density of 250×250 unit/m2. Through calculation and verification, it is found in the research that an optimal aspect ratio of a honeycomb channel is 1.5. With this structural ratio, it can be ensured that light intensity is fully used. In addition, GU Changjun et al. also researched such a structure. Different from that of LU Yuanwei et al., GU Changjun et al. used a ceramic mesh in place of a metal mesh and also achieved a desirable experimental effect.
However, the honeycomb-channel stacked photocatalyst apparatuses in the foregoing three cases also have the following disadvantage: air flowing in the channels is subject to relatively large flow resistance, which probably cannot satisfy flow resistance requirements in the field of ventilation systems that are relatively sensitive to flow resistance, for example, a ventilation system for a vehicle.
(4) Light-source-extended Type
In addition to the foregoing photocatalyst apparatuses of relatively conventional types, some other researchers tried some unconventional ways, and extended light paths to make light intensity distribution more uniform, so as to improve reaction efficiency.
FENG Qiaolian et al. proposed a concept of using optical fibres as extended light sources. A photocatalyst apparatus 1E conceived of by FENG Qiaolian et al. is shown in FIG. 5. Optical fibres LF replace blades of a fan BL and are inserted in a hub H of the fan BL. A TiO2 coating covers the surface of the optical fibres LF. An ultraviolet lamp tube 20E is disposed at the centre of the hub, and optical fibre bundles distributed in a radial direction can extend ultraviolet sources to a coating part, so as to activate a photocatalyst to set off a photocatalytic reaction. However, this concept has not been physically implemented yet, and the actual effect of the concept is still to be verified.
YE Jianren proposed a carrier in which SiO2 is used as a photocatalyst. A photocatalyst apparatus 1F produced by YE Jianren is shown in FIG. 6. In an experiment, a three-dimensional skeleton made of SiO2 needs to be prepared first, a TiO2 coating is then attached to the skeleton, and the skeleton is placed inside the apparatus in the form of a packed bed. An ultraviolet lamp tube is disposed near the carrier. Since a light path may be extended for a light source by using SiO2, a photocatalyst in such a form is also of a light-source-extended type, being significantly conducive to improvement of reaction efficiency.
As may be known from the foregoing description, there is still no such a photocatalytic apparatus that can maximally increase a contact area between polluted air and a photocatalyst, as well as maximally ensure that the surface of the photocatalyst receives sufficient irradiation of light, and can also make the flow resistance to air flowing in a channel meet requirements in the field of ventilation systems that are demanding on flow resistance, for example, the field of airplane ventilation system design. Therefore, how to design a photocatalytic apparatus that can satisfy all these conditions becomes a technical problem that urgently needs to be resolved.